At present, in order to step up the response at absorpion measurements in spectrophotometry, there is practiced repeated passage of radiation through an absorbing layer. To attain this, there are used various combinations of mirrors that transfer a light flux from a source through the absorbing medium with minimum losses. The range of practical applications of multiple reflection mirror systems is vast, including as it does both unique high-temperature units for investigating the spectra of hard-volatile compounds and serially produced spectral instruments. At present, the complete set of every infrared spectrophotometer includes a cell with an extended optical path for investigating either gases contained in extra-small concentrations, or else those of which the absorption band is very weak. Absorption cells of this kind are employed in both qualitative and quantitative analysis.
There is known a multipassage optical system (see, for instance, Journal of the Opt. Soc. of Am., 1940, Vol. 30, p. 338) comprising a mirror objective with the focal length F at one side, and two field mirrors arranged at an angle to each other--at the opposite side, and also the entrance and exit slits adjoining these mirrors. Flat mirrors are employed as field ones, with the mirror lens being positioned at the double focal length from the field mirrors.
In the aforedescribed system, as the intermediate images of the entrance slit are focused on the planar surfaces, no compression of the outermost beams takes place, whereby the multiple reflection results in a significant portion of the radiation being scattered beyond the edges of the mirror lens--the so-called inclined beam vignetting phenomenon.
There is further known a multiple reflection optical system (see, for instance, Journal of the Opt. Soc. of Am., 1942, Vol. 32, p. 285) comprising opposing reflecting members of which one is adjoined by the entrance and exit slits, and others are made up of two concave mirrors. The reflecting member adjoined by the entrance and exit slits is likewise shaped as a concave mirror, all the mirrors having the same focal length, and the spacing of the opposing mirrors being twice the focal length.
The last-described optical system of the prior art forms a system of related images on the reflecting surfaces of the mirrors. With beams making a four-run passage in the system, the first (in the beam direction) concave mirror transmits the image of the entrance slit illuminated by the source onto the surface of the opposing concave mirror. The latter forms the image of the first concave mirror on the other concave mirror, adjoining it, the last-mentioned concave mirror being used to transfer the intermediate image of the entrance slit from the aforesaid opposing concave mirror to the exit slit of the system.
By varying the angle between the concave mirrors opposing the entrance and exit slits, the sytem is tunable for a multiple (in excess of four-run) passage of the beams. However, but a slight maladjustment of the system renders the latter inoperative.
With the optical system of the prior art including similar concave mirrors arranged in opposition to the slits, the relative aperture of the system directly related to the geometric dimensions of the first concave mirror in the beam direction has proved to be inadequate for conducting an essential number of investigations in the infrared region of the operation.
It is an object of the present invention to increase the relative aperture of the system.
This object is attained in a multiple reflection optical system comprising opposing reflecting members of which one is adjoined by the entrance slit and the exit slit and others are made up of two concave mirrors in which system, in accordance with the present invention, the concave mirrors have different geometric dimensions, the reflecting member adjoining the slits being made up of two identical mirrors defining a group arranged symmetrically relative to one of the slits, the spacing between the group of the identical mirrors and the concave mirrors, as well as the focal length of the smaller concave mirror being about 1.5 times the focal length of the greater concave mirror.
In the disclosed optical system the relative aperture has been stepped up and the aberrations have been reduced. The disclosed system, furthermore, offers an optical path length which is 1.5 times that of cells with the fixed passage of the radiation and of given dimensions suited to fit the standard types of spectrophotometer. The rational arrangement of the mirrors enables the use of standard cylindrical tubes for the manufacture of the cells, instead of chambers of sophisticated configurations, which is particularly essential when the gas cells are rated for operation under gauge pressure. The disclosed optical system enables the light beams to be utilized to the fullest degree over the entire diameter of the cell, and is useful as an all-purpose multi-passage cell in all kinds of infrared spectrophotometers.